The communication of information is a necessity of modern society, which is enabled through the operation of a communication system. Information is communicated between a sending station and a receiving station by way of communication channels. The sending station, if necessary, converts the information into a form for communication over the communication channels. The receiving station detects and recovers the information for the benefit of a user. A wide variety of different types of communication systems have been developed and are regularly employed to effectuate communication between the sending and receiving stations. New types of communication systems have been and continue to be developed and constructed as a result of advancements in communication technologies.
An exemplary communication system is a radio communication system in which a communication channel is defined upon a radio link extending between the sending and receiving stations. The radio communication systems are amenable to implementation as mobile communication systems wherein radio links, rather than fixed, wireline connections, are employed to define communication channels. A cellular communication system is an example of a radio communication system that has achieved significant levels of usage. Cellular communication systems have been installed throughout significant parts of the populated world. Various cellular communication standards have been promulgated, setting forth the operational parameters of different types of cellular communication systems.
Generally, a cellular communication system includes a network infrastructure that includes a plurality of base stations that are positioned at spaced-apart locations throughout a geographic area. Each of the base stations defines an area, referred to as a cell, from which the cellular communication system derives its name. The network infrastructure of which the base stations form portions is coupled to a core network such as a packet data backbone or a public-switched telephone network. Communication devices such as computer servers, telephone stations, etc., are, in turn, coupled to the core network and are capable of communication by way of the network infrastructure and the core network. Portable transceivers, referred to as mobile stations, communicate with the base stations by way of radio links forming portions of an electromagnetic spectrum. The use of the cellular communication system is permitted, typically, pursuant to a service subscription and users (referred to as subscribers) that communicate by way of the cellular communication system through utilization of the mobile stations.
Information communicated over a radio link is susceptible to distortion such as dispersion as a result of non-ideal communication conditions. The distortion causes the information delivered to a receiving station to differ from the corresponding information transmitted by the sending station. If the distortion is significant, the informational content will not be accurately recovered at the receiving station. For instance, fading caused by multi-path transmission distorts information communicated over a communication channel. If the communication channel exhibits significant levels of fading, the informational content may not be recoverable.
Various techniques such as spatial diversity are employed to compensate for, or otherwise overcome, the distortion introduced upon the information transmitted over a communication channel to the receiving station. Spatial diversity is created through the use, at a sending station, of more than one transmit antenna from which information is transmitted, thereby creating spatial redundancy therefrom. The antennas are typically separated by distances sufficient to ensure that the information communicated by respective antennas fades in an uncorrelated manner. Additionally, the receiving stations sometimes use more than one receive antenna, preferably separated by appropriate distances.
Communication systems that utilize both multiple transmitting antennas and multiple receiving antennas are often referred to as being multiple-input, multiple-output (“MIMO”) systems. Communications in a MIMO system provide the possibility that higher overall capacity of the system, relative to conventional systems, can be achieved. As a result, an increased number of users may be serviced or more data throughput may be provided with improved reliability for each user. The advantages provided through the use of spatial diversity are further enhanced if the sending station is provided with information about the state or performance of the communication channel between the sending and receiving stations.
A sending station generally cannot measure channel characteristics of the communication channel directly, such as a channel correlation matrix representing a product of channel impulse response components for the multiple transmitting antennas. Thus, the receiving station typically measures the channel characteristics of the communication channel. In two-way communication systems, measurements made at the receiving station can be returned to the sending station to provide the channel characteristics to the sending station. Communication systems that provide this type of information to multiple-antenna sending stations are referred to as closed-loop transmit diversity systems. Communication channels extending from the network infrastructure of a cellular communication system to a mobile station are sometimes referred to as being downlink, or forward-link, channels. Conversely, the channels extending from the mobile station back to the network infrastructure are sometimes referred to as being uplink, or reverse-link, channels.
The feedback information returned to the sending station (e.g., the network infrastructure such as a base station) from the receiving station (e.g., a mobile station) is used to select values of antenna weightings. The weightings are values including phase delays by which information signals provided to individual antennas are weighted prior to their communication over a communication channel to the mobile station. A goal is to weight the information signals in amplitude and phase applied to the antennas in a manner that best facilitates communication of the information to the receiving station. The weighting values of the antenna approach a conjugate of the subspace spanned by a downlink channel covariance matrix. Estimation of the antenna weightings can be formulated as a transmission subspace tracking procedure. Several closed-loop transmit diversity procedures may be utilized.
As an example, transmit adaptive array (“TxAA”), eigenbeam former, and other techniques may be employed to advantage. Existing techniques, however, suffer from various deficiencies. For instance, the TxAA procedure fails to take into account a long-term covariance matrix of the communication channel in the selection of the antenna weightings. Additionally, the use of an eigenbeam former technique is dependent upon the number of antennas of the sending station. When the number of antennas increases, the complexity of such a technique increases rapidly.
Considering the limitations as described above, a system and method to control antenna weighting parameters for multiple antennas employed in a wireless communication system is not presently available for the more severe applications that lie ahead. Accordingly, what is needed in the art is a system that adaptively selects antenna weighting parameters and sends quantized increment vectors back to the transmitter, provides fast and global convergence to the ideal antenna weights, and employs minimal data rate to communicate the results from the receiver to the transmitter, overcoming many of the aforementioned limitations. In accordance therewith, a wireless communication system employing multiple antennas would benefit from such an adaptive arrangement without incurring unnecessary uplink bandwidth or the need to compromise signal strength at the receiving antenna.